1. Field of the Invention
The present invention relates generally to the deposition of films and more specifically to a high-rate, low-temperature method of depositing amorphous carbon films.
2. Description of the Prior Art
Amorphous carbon with diamondlike (single) bonds, as opposed to graphite (double) bonds, has a number of beneficial properties, including hardness, low coefficients of friction, chemically inertness, strength, and transparency. A unique feature of an amorphous carbon film is that it has thermal conductivity properties among the highest of any materials, including metals, at room temperatures and above, while at the same time being an electrical insulator, or at least only a semiconductor. A semiconductive amorphous carbon film has a variable band gap of about 1.0 to 5.45 eV, which is large when compared with more traditional semiconductors, such as silicon with 1.1 eV, and gallium arsenide with 1.4 eV. In spite of such a large band gap, amorphous carbon films are being considered for use as semiconductors, because they have a rather smooth increase in velocity of current-carrying free electrons in response to electric fields.
Because of these and other properties, more and different uses for carbon films are being found for applications in tribology, electronics, mechanical systems, and even semiconductors. Such applications include, for example, wear resistant coatings for gears, protective and transparent coatings on windows and optical equipment, high-temperature electronic components, tool coatings for improved wear and hardness, and abrasives for grinding pads and wheels.
Despite a great deal of excitement generated by the myriad potential applications of carbon films, there remain several obstacles to large-scale commercial production and use. The most significant of such obstacles include high deposition temperatures, high costs, and relatively slow deposition rates. Typically, the deposition temperatures of carbon films can be in the range of 1000.degree. to 4000.degree. C., although lower deposition temperatures are also used. Examples of such high-temperature processes may be found in U.S. Pat. No. 3,549,847 issued to Clark et al., U.S. Pat. No. 3,924,034 issued to Olcott, U.S. Pat. No. 4,701,317 issued to Arakawa et al., and U.S. Pat. No. 4,825,049 issued to Rickborn et al. Yet deposition rates at these high temperature levels are only about 100 .ANG./min.
There have been several attempts to increase deposition rates of these high temperature processes. For example, the article, "Is Diamond the New Wonder Material?" Science, Vol. 234, pgs. 1074-1076, 28 Nov. 1986, reported high-temperature deposition rates as high as 10 .mu.m/hr (micrometers per hour), or about 1600 .ANG./min, of polycrystalline carbon film at the Nippon Institute of Technology in Saitama, Japan, with a high-temperature, tungsten filament method using acetone as a source material.
U.S. Pat. No. 3,925,577, issued to Fatzer et al. shows a process for coating isotropic graphite with a silicon carbide layer, which is done by depositing a layer of silicon on graphite and then heating to melt the silicon into the graphite. The first step in this process includes heating the graphite to a very high temperature, e.g., 1700.degree. C. to 2400.degree. C., in a halogen atmosphere of chlorine or fluorine to reduce the impurities.
The article, "Study on Hydrophobic a-C:H:F Overcoat Layer for a-Si:H Photoreceptor" by F. Ishikawa et al., Materials Research Society Symposium Proceedings, Vol. 118, pgs. 429-434, 1988, disclosed an amorphous hydrogenated fluorocarbide film used to replace a hydrogenated amorphous silicon carbide (a-SiC:H) film as a passive overcoat on photoreceptors used in electrophotography to increase blurring lifetime, the deposition process of which involved plasma-enhanced chemical vapor deposition (PE-CVD) of perfluoroethane (C.sub.2 F.sub.6) and hydrogen (H.sub.2). However, the deposition rate and temperature described by Ishikawa, et al., were within ordinary, commonly known carbon film deposition parameters.
Some of the special problems associated with these conventional carbon film deposition methods include higher costs of production, especially in some processes due to special high-temperature furnaces and other equipment and increased power demands required to generate such heat. However, the most important limitation imposed by such high-temperature carbon film deposition processes is that the choice of substrates onto which the carbon films can be deposited is severely restricted to only materials that withstand the high temperatures. Yet, there are many ordinary, inexpensive materials, the usefulness and durability of which could be greatly enhanced by carbon films. For example, most conventional plastics melt, burn, or otherwise break down in the high temperatures utilized in such carbon film deposition processes as those described above.
Consequently, more efficient low-temperature (i.e., room temperature) carbon film deposition processes would be very beneficial, and several have been under investigation.
For example, U.S. Pat. No. 4,504,519, issued to Zelez and U.S. Pat. No. 4,756,964, issued to Kincaid et al. show such low-temperature carbon film deposition methods. However, these low-temperature processes typically have deposition rates in the range of only about 10 .ANG./min. Accordingly, they have been used in only special applications, such as coating plastics that can not tolerate high temperatures.
In spite of all the attempts to solve this problem over the past several years, no one has yet been able to develop a process for the deposition of amorphous carbon films that includes the cost saving features of both higher deposition rates and lower deposition temperatures. Equally as important, there is a need for a low-temperature, high deposition rate process that also preserves the original properties and molecular purities of the resulting amorphous carbon film.